1. Field of the Invention
The present invention relates to an electrically driven vehicle such as, for example, a battery operated automobile or a fuel cell powered automobile utilizing a combination of fuel and battery and, more particularly, to a motor control device for the electric automobile of a type utilizing an interior permanent magnet synchronous motor.
2. Description of Related Art
In an electric automobile, reduction in performance and failure of a motor used to drive the automobile significantly affect the roadability and the safety. In particular, with an electric automobile drive device of a battery drive type, an embedded permanent magnet synchronous motor (also referred to as an IPM type DC brushless motor) utilizing a neodymium magnet and having a high efficiency performance has been utilized in order to increase the cruising distance with a limited available battery capacity. Also, in the field of an in-wheel motor device, the system has hitherto been suggested in which for the purpose of securement of the reliability, the temperature of, for example, a motor is measured to monitor the excessive load and the electric drive current for the motor is limited or the motor rotation number (motor number of revolutions or motor speed) is reduced in dependence on the measured temperature (such as disclosed in, for example, the patent documents 1 to 3 listed below).    [Patent Document 1] JP Laid-open Patent Publication No. 2006-258289    [Patent Document 2] JP Laid-open Patent Publication No. 2004-328819    [Patent Document 3] JP Laid-open Patent Publication No. 2008-168790
In the motor used for the electric automobile, particularly the motor of a type utilizing the neodymium permanent magnet, the permanent magnet has a low heat resistance temperature and, therefore, if the ambient temperature exceeds the heat resistance temperature, the irreversible demagnetization occurs. Accordingly, the motor driving capability abruptly decreases and, depending on the circumstances, the automobile will become unable to control. Although as described above, in the in-wheel motor device, the motor temperature is measured to monitor the excessive load and, if required, the drive is limited, the control relying on the result of measurement of the motor temperature is incapable of taking any countermeasure against the reduction in performance brought about by the demagnetization of the permanent magnet used in the motor.
In view of the foregoing, the present invention has for its primary object to provide a motor control device for an electric automobile, which device is capable of suppressing a reduction of the motor driving force in the event of the occurrence of demagnetization in a permanent magnet used in a motor.
Another important object of the present invention is to provide the electric automobile of the type referred to above, which can run by its own bootstraps, i.e., by its resources, to the nearest place where the maintenance servicing is available, with the reduction in motor driving force being prevented, in the event of the occurrence of demagnetization in a permanent magnet used in a motor.
The summary of the present invention will be hereinafter described with the aid of reference numerals shown in the accompanying drawings and used for facilitating a better understanding of preferred embodiments of the present invention.